In the proposed project our objectives are: 1. To develop a more suitable culture system for JC virus, a human papovavirus. 2. To continue studies of the role of JC virus, and to a lesser extent BK virus, in human infection and disease. Much of the work will be concentrated on the epidemiology of BK virus and JC virus and on JC virus in progressive multifocal leukoencephalopathy. To continue investigations of the oncogenicity of JC virus.